Manipulating anion solvation competitiveness via a multifunctional additive toward robust low-temperature sodium metal batteries

Abstract

The kinetic conflict between accelerated Na⁺ desolvation and durable interphase construction remains a bottleneck for sodium metal batteries (SMBs) operating at low temperatures. Herein, we propose an electrolyte engineering strategy employing 0.20 wt.% pentafluoro(phenoxy)cyclotriphosphazene (FPPN) as a solvation-modulated additive, which concurrently optimizes Na+-desolvation energetics and directs self-assembling interphase architectures. Through precisely engineered coordination competition between PF6⁻ anions and diethylene glycol dimethyl ether (DG) molecules, FPPN promotes the formation of a bifunctional solid electrolyte interphase (SEI) comprising an ion-conductive NaF-dominant inner stratum and a mechanically resilient fluorocarbon outer layer. This hierarchically structured SEI enables spatially homogeneous Na⁺ flux distribution while maintaining exceptional interfacial cohesion. The derived cathode electrolyte interphase (CEI) exhibits superior anodic stability through anion-derived inorganic reinforcement. Consequently, the Na||Na symmetrical cell exhibits an extraordinary cycle life of 1400 h at -40 °C. When paired with Na3V2(PO4)3, the cell sustains an impressive 3300 cycles at 1 C and -20 °C. Even cycled at -40 °C, a high capacity of 60 mAh g-1 after 1750 cycles with a remarkable retention ratio of 98.00% can be realized. This work offers pivotal insights for designing electrolytes to achieve extended cycle life in low-temperature rechargeable metal batteries.